1. Field of the Invention
The present invention relates to equipment used in boring into terrestrial formations. More specifically, the present invention relates to bits and cutters for bits used to drill well bores into the earth for use in the recovery of hydrocarbons and other minerals.
2. Description of the Related Art
The equipment used to drill well bores in the earth for the extraction of hydrocarbons has included a variety of bits and bit cutter configurations intended to penetrate specific formations. The bits are generally either of a fixed cutter design or a roller cone design, with each design having its own benefits and advantages as applied to a particular drilling operation. The cutting action of the bit requires it to be rotated into the formation or, in the case of percussion bits, to be repeatedly impacted against the formation.
In typical bit designs, but particularly in the fixed cutter bit designs, the cutters are provided with a layer of super-hard material, such as a polycrystalline diamond carried on a softer substrate, such as tungsten carbide. As used herein, the term "super-hard material" is intended to include material that is harder than the supporting substrate, but specifically material such as a polycrystalline diamond. Other super-hard materials are also commonly employed for cutters. The substrate material is generally a cemented tungsten carbide but may be comprised of other materials. Examples of materials suitable for use as super-hard materials and substrate materials may be found in U.S. Pat. Nos. 4,679,639; 5,096,465; 5,111,895; and 4,766,040.
The diamond layer of the cutter is provided to enhance the cutting characteristics and longevity of the cutter. The methods of applying these super-hard layers to the cutter substrate and the mounting of the composite cutter body to a bit, as well as the materials employed for the cutters and bits, are the subject of a large number of patents and an extensive body of complex technology. Generally, the various super-hard materials and substrates used in the manufacture of cutters for bits are well known and, per se, form no part of the present invention. The methods for bonding the substrate and super-hard materials to each other for mounting the cutter to the bit body are also well known and are not, per se, a part of the present invention.
The history of the development of cutter fabrication and bit design is replete with examples of significant benefit deriving from only a small change in an existing cutter face design or composition of material, or even a method of fabricating the components of the bit. In some cases, the technical basis for the improved results stemming from these small changes is not well understood. The evidence of the improvement is seen in such objective criteria as an increased rate of penetration, a reduction in bit wear, a longer bit life, a reduction in the cost of manufacture, or other similar result deriving from the improvement.
The cutting face on the cutter element itself is also the subject of intensive design and engineering effort. Cutting faces on the cutters of fixed cutter bits, as well as roller cone bits, have assumed a variety of different configurations, each with one or more special features intended to improve the quality of the bit's drilling action. A large number of the prior art cutter faces employ a planar diamond surface or table carried at the end of a cylindrical tungsten carbide mounting body. The cross-sectional profile of the planar surface is often circular or may be oblong, the latter form generally being referred to as a "tombstone" cutter. Generally, the cutting face, which is intended to engage the uncut formation, is mounted on the bit such that the plane of the cutting face is angled relative to the direction of the cutter rotation. If the face plane is angled away from the direction of rotation, the cutter is said to have a negative rake. A cutter face normal to the direction of bit rotation has a zero rake, and a face angled into the direction of bit rotation has a positive rake. Cutter faces that are inclined laterally relative to the direction of cutter rotation are said to have a side rake.
It is also common to provide a curving, rather than a planar, cutting face on the cutter. Concave curving faces on fixed cutter bits are illustrated, for example, in U.S. Pat. Nos. 4,538,690; 4,558,753; 4,593,777; 4,679,639; 5,025,874; 5,078,219; 5,101,691; 5,377,773; and 5,460,233. A recognized feature of the curving cutting face is that a single curved surface can provide a variable rake angle along the cutting surface of the face.
U.S. Pat. No. 5,706,906 (the '906 patent) illustrates a variety of cutting faces that are curving, planar, concave, or convex, and various combinations thereof. The cutter faces described in the '906 patent are generally oriented on the bit to direct cutting forces toward the center of the cutter in the area of the longitudinal axis of the cutter that extends generally transversely to the plane of the cutting face. The wear pattern of the '906 cutters generally extends from the leading cutting face to the cylindrical side of the cutter. The cutting faces of the cutters of the '906 Patent are described for use in a conventional mounting orientation on the bit with the central axis of the cutter being positively inclined so that the cutter mount pushes the cutting face into the formation. In general, a major portion of the diamond in the '906 cutters is positioned ahead of the point of engagement of the cutter with the formation.
U.S. Pat. No. 4,570,726 (the '726 patent) illustrates a cutter having a cutting face with a negative rake angle formed primarily along the side of a cylindrical support or shank. The wear pattern of the cutter extends from the leading cutting face along the cutter side toward the axial cutter end. One form of the cutting face is a partial cylindrical wall that extends generally parallel to the axis of the cutter. Other forms show a relatively complex working or cutting surface that is non-parallel to the axis. The shank is secured to the cutter such that the cutter face has a negative rake angle and a curved contact area for engaging the formation. An abrasive substance is deposited over the contact area but is not deposited on the free axial end surface of the cutter. The cutter face of the '726 patent is described as being either symmetrical or nonsymmetrical, as desired, for a particular application. The formation contact portion of one embodiment of the '726 patent is described as having a leading part that has a convex cross-section in one plane with side parts having cross-sections that are partially convex and partially concave. The cutter is described as having improved material flow and strain features.
The '726 patent describes a cutter in which the interface between the abrasive material and the supporting substrate forms an edge of the cutting surface that acts as a self-sharpening edge. This design, while effective in maintaining a sharp cutting edge as the bit wears, sacrifices bit life and design flexibility for cutting efficiency. The edge exposed to the uncut formation is also more prone to chipping or spauling of the super-hard abrasive layer as the underlying substrate wears away. Impact resistance of a self-sharpening cutter face is generally also not as good as that expected from a cutter face that is comprised exclusively of super-hard material. The requirement for a substrate-to-abrasive material interface in the cutting face also reduces the design flexibility for providing relatively large volumes of diamond in the wear area of the cutter.
Cost is an important consideration in the fabrication of bit cutter elements. Generally, the more complex the cutter surface, the more difficult and expensive it is to fabricate the cutter. It is also generally true that a nonsymmetrical cutter face is more complex and thus more expensive to produce than a symmetrical face. Diamond cutters are usually formed in a press to shape and bond the diamond and substrate materials. Complex diamond cutting surfaces, however, are not easily formed in the pressing process. Where a complex shape is required, it is usually necessary to cut the shape with an electrical discharge machining process or to machine the desired shape from a pressed symmetrical diamond cutting surface. The machining step adds cost to the fabrication of the final cutter. Any cutting face design that may be pressed into the diamond rather than being machined is generally less expensive to fabricate. Complex designs, such as the geometric shapes described in the '726 patent, are difficult to form in a press and, to the extent that they are not capable of being turned on a lathe or centerless grinder, are equally difficult to machine.